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| CMS-TOP-16-016 ; CERN-EP-2017-023 | ||
| Search for standard model production of four top quarks in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 20 February 2017 | ||
| Phys. Lett. B 772 (2017) 336 | ||
| Abstract: A search for events containing four top quarks (${\mathrm{ t \bar{t} }\mathrm{ t \bar{t} }} $) is reported from proton-proton collisions recorded by the CMS experiment at $\sqrt{s} = $ 13 TeV and corresponding to an integrated luminosity of 2.6 fb$^{-1}$. The analysis considers the single-lepton (e or $\mu$)+jets and the opposite-sign dilepton ($\mu\mu$, $\mu^{\pm} \mathrm{ e }^{\mp}$, or $\mathrm{ e }^{+}\mathrm{ e }^{-}$)+jets channels. It uses boosted decision trees to combine information on the global event and jet properties to distinguish between ${\mathrm{ t \bar{t} }\mathrm{ t \bar{t} }} $ and $\mathrm{ t \bar{t} }$ production. The number of events observed after all selection requirements is consistent with expectations from background and standard model signal predictions, and an upper limit is set on the cross section for ${\mathrm{ t \bar{t} }\mathrm{ t \bar{t} }} $ production in the standard model of 94 fb at 95% confidence level (10.2 $\times$ the prediction), with an expected limit of 118 fb. This is combined with the results from the published CMS search in the same-sign dilepton channel, resulting in an improved limit of 69 fb at 95% confidence level (7.4 $\times$ the prediction), with an expected limit of 71 fb. These are the strongest constraints on the rate of ${\mathrm{ t \bar{t} }\mathrm{ t \bar{t} }} $ production to date. | ||
| Links: e-print arXiv:1702.06164 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures & Tables | Summary | Additional Figures & Tables | References | CMS Publications |
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| Figures | |
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Figure 1:
A representative Feynman diagram for ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ production in the SM at lowest order in QCD. |
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Figure 2:
Distribution of the event-level BDT discriminants ${D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\mathrm {lj}}}$ for the $\mu $+jets (left) and e+jets (right) final states from data and the estimated background contributions from simulation, in the $ N_{\mathrm {j}} \geq $ 9 and 3 $ N_{\text {tags}}^{\mathrm {m}} $ (upper panels) and the $ N_{\mathrm {j}} \geq $ 9 and $\geq $ 4 $ N_{\text {tags}}^{\mathrm {m}} $ categories (lower panels). The vertical bars show the statistical uncertainties in the data. The predicted background distributions from simulation are shown by the shaded histograms The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the matrix-element (ME) factorization and renormalization scales used in the simulation. The expected SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ signal contribution is shown by open histogram, multiplied by a factor of 20. |
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Figure 2-a:
Distribution of the event-level BDT discriminants ${D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\mathrm {lj}}}$ for the $\mu $+jets final state from data and the estimated background contributions from simulation, in the $ N_{\mathrm {j}} \geq $ 9 and 3 $ N_{\text {tags}}^{\mathrm {m}} $ category. The vertical bars show the statistical uncertainties in the data. The predicted background distributions from simulation are shown by the shaded histograms The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the matrix-element (ME) factorization and renormalization scales used in the simulation. The expected SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ signal contribution is shown by open histogram, multiplied by a factor of 20. |
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Figure 2-b:
Distribution of the event-level BDT discriminants ${D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\mathrm {lj}}}$ for the e+jets final state from data and the estimated background contributions from simulation, in the $ N_{\mathrm {j}} \geq $ 9 and 3 $ N_{\text {tags}}^{\mathrm {m}} $ category. The vertical bars show the statistical uncertainties in the data. The predicted background distributions from simulation are shown by the shaded histograms The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the matrix-element (ME) factorization and renormalization scales used in the simulation. The expected SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ signal contribution is shown by open histogram, multiplied by a factor of 20. |
png pdf |
Figure 2-c:
Distribution of the event-level BDT discriminants ${D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\mathrm {lj}}}$ for the $\mu $+jets final state from data and the estimated background contributions from simulation, in the $ N_{\mathrm {j}} \geq $ 9 and $\geq $ 4 $ N_{\text {tags}}^{\mathrm {m}} $ category. The vertical bars show the statistical uncertainties in the data. The predicted background distributions from simulation are shown by the shaded histograms The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the matrix-element (ME) factorization and renormalization scales used in the simulation. The expected SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ signal contribution is shown by open histogram, multiplied by a factor of 20. |
png pdf |
Figure 2-d:
Distribution of the event-level BDT discriminants ${D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\mathrm {lj}}}$ for the e+jets final state from data and the estimated background contributions from simulation, in the $ N_{\mathrm {j}} \geq $ 9 and $\geq $ 4 $ N_{\text {tags}}^{\mathrm {m}} $ category. The vertical bars show the statistical uncertainties in the data. The predicted background distributions from simulation are shown by the shaded histograms The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the matrix-element (ME) factorization and renormalization scales used in the simulation. The expected SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ signal contribution is shown by open histogram, multiplied by a factor of 20. |
png pdf |
Figure 3:
Distribution of the event-level BDT discriminants ${D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\text {dil}}}$ for the combined dilepton ($\pi \mu $+ $\mu ^{\pm } \mathrm{ e } ^{\mp }$+ $\mathrm{ e }^{+} \mathrm{ e }^{-} $) event sample for 4-5 jets (upper left ), 6-7 jets (upper right ), and $\geq $8 jets (bottom). The vertical bars show the statistical uncertainty in the data. The predicted background distributions from simulation are shown by the shaded histograms. The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the choice of the matrix-element (ME) factorization and renormalization scales used in the simulation. The electroweak (EW) histogram is the sum of the Drell-Yan and W boson+jets backgrounds. The expected SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ signal contribution is shown by the open histogram, multiplied by a factor of 20. |
png pdf |
Figure 3-a:
Distribution of the event-level BDT discriminants ${D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\text {dil}}}$ for the combined dilepton ($\pi \mu $+ $\mu ^{\pm } \mathrm{ e } ^{\mp }$+ $\mathrm{ e }^{+} \mathrm{ e }^{-} $) event sample for 4-5 jets. The vertical bars show the statistical uncertainty in the data. The predicted background distributions from simulation are shown by the shaded histograms. The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the choice of the matrix-element (ME) factorization and renormalization scales used in the simulation. The electroweak (EW) histogram is the sum of the Drell-Yan and W boson+jets backgrounds. The expected SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ signal contribution is shown by the open histogram, multiplied by a factor of 20. |
png pdf |
Figure 3-b:
Distribution of the event-level BDT discriminants ${D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\text {dil}}}$ for the combined dilepton ($\pi \mu $+ $\mu ^{\pm } \mathrm{ e } ^{\mp }$+ $\mathrm{ e }^{+} \mathrm{ e }^{-} $) event sample for 6-7 jets. The vertical bars show the statistical uncertainty in the data. The predicted background distributions from simulation are shown by the shaded histograms. The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the choice of the matrix-element (ME) factorization and renormalization scales used in the simulation. The electroweak (EW) histogram is the sum of the Drell-Yan and W boson+jets backgrounds. The expected SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ signal contribution is shown by the open histogram, multiplied by a factor of 20. |
png pdf |
Figure 3-c:
Distribution of the event-level BDT discriminants ${D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\text {dil}}}$ for the combined dilepton ($\pi \mu $+ $\mu ^{\pm } \mathrm{ e } ^{\mp }$+ $\mathrm{ e }^{+} \mathrm{ e }^{-} $) event sample for $\geq $8 jets. The vertical bars show the statistical uncertainty in the data. The predicted background distributions from simulation are shown by the shaded histograms. The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the choice of the matrix-element (ME) factorization and renormalization scales used in the simulation. The electroweak (EW) histogram is the sum of the Drell-Yan and W boson+jets backgrounds. The expected SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ signal contribution is shown by the open histogram, multiplied by a factor of 20. |
| Tables | |
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Table 1:
Expected and observed 95% CL upper limits on SM ${{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } }$ production as a multiple of ${\sigma _{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\mathrm {SM}}}$ and in fb. The results for the two analyses from this paper are shown separately and combined. The result from a previous CMS measurement [12] is also given, along with the overall limits when the three measurements are combined. The values quoted for the uncertainties on the expected limits are the one standard deviation values and include all statistical and systematic uncertainties. |
| Summary |
| In summary, a search has been performed for events containing four top quarks using data recorded by the CMS experiment in proton-proton collisions at $\sqrt{s} = $ 13 TeV corresponding to an integrated luminosity of 2.6 fb$^{-1}$. The final states considered in the analysis are the single-lepton channel with exactly one electron or muon, and the opposite-sign dilepton channel with exactly two of any combination of electrons or muons. A boosted decision tree is used to discriminate between the ${\mathrm{ t \bar{t} }\mathrm{ t \bar{t} }} $ signal and the $\mathrm{ t \bar{t} }$ background, and no signal is observed. This leads to an upper limit on the SM production cross section for$ {\mathrm{ t \bar{t} }\mathrm{ t \bar{t} }} $ of 94 fb (10.2 ${\sigma_{{\mathrm{ t \bar{t} }\mathrm{ t \bar{t} }} }^{\mathrm{SM}}} $), with an expected limit of 118$^{+76}_{-41}$ fb at the 95% confidence level. This result is combined with a previous search [12] with similar sensitivity in the same-sign dilepton channel to obtain an improved limit of 69 fb, with an expected limit of 71$^{+38}_{-24}$ fb. This is the most stringent limit on $ {\mathrm{ t \bar{t} }\mathrm{ t \bar{t} }} $ production at $\sqrt{s } = $ 13 TeV published to date. |
| Additional Figures | |
png pdf |
Additional Figure 1:
Summary of the event-level BDT discriminant ${ {D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\mathrm {lj}}} }$ for the single electron events, showing the jet multiplicities and number of b jets considered in the analysis, before the fit. The vertical bars show the statistical uncertainty in the data. The predicted background distributions from simulation are shown by the shaded histograms. The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the choice of the matrix-element (ME) factorization and renormalization scales used in the simulation. The electroweak (EW) histogram is the sum of the Drell-Yan and W boson+jets backgrounds. The expected SM $ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } $ signal contribution is shown by the open histogram, multiplied by a factor of 20. |
png pdf |
Additional Figure 2:
Summary of the event-level BDT discriminant ${ {D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\mathrm {lj}}} }$ for the single muon events, showing the jet multiplicities and number of b jets considered in the analysis, before the fit. The vertical bars show the statistical uncertainty in the data. The predicted background distributions from simulation are shown by the shaded histograms. The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the choice of the matrix-element (ME) factorization and renormalization scales used in the simulation. The electroweak (EW) histogram is the sum of the Drell-Yan and W boson+jets backgrounds. The expected SM $ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } $ signal contribution is shown by the open histogram, multiplied by a factor of 20. |
png pdf |
Additional Figure 3:
Summary of the event-level BDT discriminant ${ {D_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }^{\mathrm {dil}}} }$ for the dilepton ($\mu ^+\mu ^-$+ $\mu ^{\pm } {\rm e}^{\mp }$+ ${\rm e}^+ {\rm e}^-$) events, showing the jet multiplicities considered in the analysis, before the fit. The vertical bars show the statistical uncertainty in the data. The predicted background distributions from simulation are shown by the shaded histograms. The hatched area shows the size of the dominant systematic uncertainty in the simulation, which comes from the choice of the matrix-element (ME) factorization and renormalization scales used in the simulation. The electroweak (EW) histogram is the sum of the Drell-Yan and W boson+jets backgrounds. The expected SM ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ signal contribution is shown by the open histogram, multiplied by a factor of 20. |
png pdf |
Additional Figure 4:
Summary of the expected and observed upper limits on $ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } $ production, in multiples of the standard model $ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } $ cross section, $\sigma ^{SM}_{ {{\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } } }$, for the SS dilepton, OS dilepton, single lepton, and combined analyses. |
| Additional Tables | |
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Additional Table 1:
Number of observed and expected background events after preselection in each search channel. |
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